Ceramic resistor card assemblies have been utilized in fuel level indication systems heretofore. The ceramic material and the inks printed thereon are stable when exposed to harsh elements within a gasoline fuel tank. There are several standard designs.
In a first standard design, contacts located on a wiper assembly form a shorting bar across a resistor on the ceramic card and a conductor on the ceramic card. A prior art ceramic resistor card 10 is shown in FIG. 1. A wiper button makes contact at designated intervals with conductive bars 12 emanating from a resistive ink 14 printed over the conductive bars. A second wiper button rides on a flat continuous conductive path 16 screened onto the ceramic substrate. An alloy of copper, zinc and nickel known as "silver nickel" is used as a material for the contacts due to the relative softness of this material and its relative low cost. Substantial material volume must be utilized to allow for the significant contact wear to insure part durability.
In a second standard design, contacts located on the wiper assembly form a shorting bar across a resistor on a ceramic card and a metal conductor plate. The wiper contact button makes contact at designated intervals with conductive bars emanating from underneath the resistor ink. The second contact button rides on the metal plate.
Both of these standard designs are susceptible to the buildup of contact resistance. Contact resistance can change the output value of the resistor assembly or in some instances can cause "open circuit" conditions. "Silver nickel" has a tendency to oxidize under environmental conditions found in today's gasoline fuel tanks, and with an increased use of oxygenated fuels. To overcome contact resistance, many designs rely on voltage in the circuit and the "scrubbing" action of the contacts riding across the surface they wipe on. "Scrubbing" is mechanical abrasion, which is the interaction of the contact surfaces and the force with which the contacts are held against the surfaces. With the designs described above, relatively high forces in the 20 to 40 gram range are used to cause abrasion or wear against the ink/ceramic surface or the metal contact plate. Contact wear is further aggravated by the presence of the fuel. The fuel washes the abrasive surfaces, thereby "renewing" the cutting surfaces of the abrasives in the ceramics and the inks. In contrast, the contact wear under dry conditions creates "smooth" surfaces as surface irregularities are filled in with abraded material. "Burn through" is achieved when sufficient voltage is applied to the circuit to overcome the contact resistance. The designs described above require a voltage which can produce 25 mA to overcome contact resistance on a consistent basis.
The change in electro-mechanical gauges to electronic modules for the display of fuel levels has reduced the voltage provided to the variable resistor from 13.5 volts to 5 volts. Pull-up resistors used in these electronic modules or computers further decrease the voltage so that only 10 mA are available. Since the designs described above require 25 mA to consistently override resistance they cannot be used in these electronic systems.
The recent implementation of onboard diagnostic evaporative emission requirements force the fuel indication system to be more accurate. Any problems resulting from contact resistance may jeopardize the accuracy of the systems and their ability to meet onboard diagnostic requirements.
The present invention provides advantages over the prior art.